Recently, ceramics composed principally of silicon nitride (Si.sub.3 N.sub.4) have found significant use as ceramic components for machines or as vessel coatings. This material is known to have many good characteristics at high temperatures (1200.degree.-1400.degree. C.), e.g., good oxidation resistance, good mechanical strength, and good hardness.
The preferred method for making silicon nitride comprising ceramics of high density and high strength has been by a process known in the art as hot pressing. However, in spite of the use of hot pressing, the bend strength of simple Si.sub.3 N.sub.4 has not been as high as desired at high temperatures. Accordingly, other avenues of strength improvement have been sought such as through the use of additives which operate as densification aids while not significantly impairing the creep resistance of the ceramic body at high temperatures. These added materials have included relatively large amounts of chromium oxide, zinc oxide, nickel oxide, titanium oxide, cerium oxide, magnesium oxide, yttrium oxide and others, ranging in excess of 20% (wt.) of the matrix material. Silicon nitride with these particular additives tends to form a structure having a strength level which does not usually exceed 50 KSI at high temperatures. In one instance (U.S. Pat. No. 3,830,652 to Gaza) the prior art did report strength levels in excess of 50 KSI. In this instance, the concern was for physical characteristics useful for turbine elements: hardness, oxidation resistance (inertness) and transverse rupture strength. Gaza explored metal oxide additives to a Si.sub.3 N.sub.4 system which ranged in amounts related solely to machine element usage. The additions were added in amounts up to 20%.
However, commercial cutting tools used today and prepared from materials other than silicon nitride exhibit the same or better physical properties than the silicon nitride based materials which were the focus of Gaza's work. For example, commercial Al.sub.2 O.sub.3 or TiC tools have excellent hardness at high temperatures. They also have high resistance to oxidation and have transverse rupture strengths at high temperatures which range up to 100,000 psi. Strength has heretofore been considered the most important feature of tools because of the necessity to withstand forces imposed on the tool material by the tool fixture and by the resistance of the stock material, particularly at heavy depths of cutting. These forces become unusually exaggerated when cutting ferrous material such as cast iron at high speeds and feeds. Without increased strength, it is believed by those skilled in the art that further improvements in tool life cannot be achieved. Since the strength level of Si.sub.3 N.sub.4 is equal to or lower than commercial materials now available, it has been rejected as a tool material candidate with little hope of improving tool life.
In only one known instance has the art attempted to employ Si.sub.3 N.sub.4 directly as a cutting tool material and this was for use only on hypereutectic aluminum alloys. This attempt is set forth in Japanese Pat. No. 49-113803 (Oct. 30, 1973) by Kazutaka Ohgo, appearing in Chemical Abstracts, Volume 84, 1976, page 286 (84:21440t). In this work, silicon nitride was sintered and metal oxide spinels were employed in solid solution in the silicon nitride matrix. The spinels were formed by a mixture of divalent and trivalent metal oxides (including magnesium oxide and Y.sub.2 O.sub.3). However, the molar percentage of the spinel metal oxide in the material was taught to be 10-40%. The author experienced difficulty in obtaining good sintering density when the molar percentage fell below 10. The highest density achieved was 3.18 g/cm.sup.3.
A two step method was used by Ohgo requiring first a heating of the metal oxide powders to 1300.degree.-1600.degree. C. for 3-10 hours to form the spinel. The spinel was pulverized and mixed with a silicon nitride powder, which in turn was sintered to form cutting tools. Only a quarternary system was employed involving Si.sub.3 N.sub.4, SiO.sub.2, MgO and Y.sub.2 O.sub.3. This produced many secondary phases which weakened the physical characteristics, particularly strength, thermal conductivity, and increased the thermal coefficient of expansion. A loss of these physical characteristics make it most difficult to obtain even equivalent performance to commercially available tools when applied to a rigorous cutting environment such as interrupted cutting on cast iron.
The aluminum alloy cutting operation used by Ohgo was of very short duration (2 minutes) of continuous machining. This type of test, of course, did not investigate cutting applications where large forces are applied to the tool, did not investigate the elimination of spinel additives, did not investigate heavy cutting against rough surfaces such as cast iron, nor continuous cutting for periods of several hours or greater, nor did it explore intermittent, interrupted high speed cutting at speeds of 4000-5000 sfm at heavy feeds and depths of cutting. The demonstrated wear of 0.006-0.008 inches, in Ohgo's work, for 2 minutes of cutting time is highly excessive when compared to the goals of the present invention. Therefore, this work did not demonstrate that Si.sub.3 N.sub.4 possessed sufficient characteristics to be used as a tool material on ferrous materials which apply large bend forces to the tool.
Moreover, the art has been possessed of sufficient knowledge in the making of Si.sub.3 N.sub.4 with additives for many years; during this long term no effort was made to apply this material as a cutting tool against cast iron. This tends to support the contention of this invention that if tool life is dramatically increased for certain Si.sub.3 N.sub.4 composites when used for machining cast iron, there must be some unobvious characteristics independent of strength that remained undiscovered to promote this new use. The attainable hardness level and general rigidity of the known silicon nitride composites have yet to be comparable to commercial cutting tools. Investigators have failed to perceive this interplay of physical characteristics. Known silicon nitride compositions, when used as a cutting tool against relatively rough surfaces such as cast iron, exhibit a failure mode under such circumstances that is typically due to thermal shock as opposed to the more desirable mode by wear.